Derivative Instruments and Hedging Activities (Fair Value of Derivative Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Asset Fair Value	$ 2,299	$ 0
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Asset Fair Value	13,868	406
Liability Fair Value	(47,084)	(29,028)
Net Derivative Fair Value	(33,216)	(28,622)
Not Designated as Hedging Instrument \| Current assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	11,569	406
Not Designated as Hedging Instrument \| Current liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(35,948)	(27,744)
Not Designated as Hedging Instrument \| Long-term assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	2,299
Not Designated as Hedging Instrument \| Long-term liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(11,136)	(1,284)
Not Designated as Hedging Instrument \| Natural gas \| Current assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	391	406
Not Designated as Hedging Instrument \| Natural gas \| Current liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(35)	0
Not Designated as Hedging Instrument \| Natural gas \| Current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	356	406
Not Designated as Hedging Instrument \| Natural gas \| Long-term assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	0	0
Not Designated as Hedging Instrument \| Natural gas \| Long-term liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(302)	0
Not Designated as Hedging Instrument \| Natural gas \| Non-current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	(302)	0
Not Designated as Hedging Instrument \| Oil \| Current assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	11,178	0
Not Designated as Hedging Instrument \| Oil \| Current liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(35,913)	(27,744)
Not Designated as Hedging Instrument \| Oil \| Current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	(24,735)	(27,744)
Not Designated as Hedging Instrument \| Oil \| Long-term assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	2,299	0
Not Designated as Hedging Instrument \| Oil \| Long-term liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(10,834)	(1,284)
Not Designated as Hedging Instrument \| Oil \| Non-current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	$ (8,535)	$ (1,284)

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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